The efficiency of thermoelectric materials and devices is determined by the dimensionless figure of merit (ZT), defined as ZT=(S2σ/κ)T, where S, σ, κ, and T are the Seebeck coefficient, the electrical conductivity, the thermal conductivity, and the absolute temperature, respectively1. The numerator product (S2σ) is called the power factor (PF). The well known interdependence of S, σ and κ, complicates efforts in developing strategies for improving a material's average ZT well above 2.5, especially using less expensive, more earth-abundant materials,2, 3, 4 a feat that could revolutionize the field of thermal energy conversion. Several approaches to enhance ZT have emerged in the last decade including; modifying the band structure5, heavy valence (conduction) band convergence6, 7, quantum confinement effects8 and electron energy barrier filtering9 to enhance Seebeck coefficients; nanostructuring10, all-scale hierarchical architecturing11, and band energy alignment between nano-precipitate/matrix to maintain hole mobility12, 13, 14. Most of these approaches aim to maintain a high power factor and reduce the lattice thermal conductivity. Alternatively, one can seek high performance in pristine thermoelectric materials with intrinsically low thermal conductivity, which may arise from a large molecular weight15, a complex crystal structure16 and charge density wave (CDWs) distortions17.
Recent years have witnessed the steady upward march of the thermoelectric figure of merit ZT. Enhancing ZT across a wide range of temperatures means enhancing the conversion efficiency of heat to electricity for a large number of potential applications. However, many of the advances have focused on improvements in the maximum ZT (ZTmax) as a function of temperature, and they have been achieved from the emergence of new concepts which enabled the huge reduction in lattice thermal conductivity (e.g., nano structuring).